CN116635206A

CN116635206A - Screw and twin screw assembly for use in an elastomeric mixture extruder and related methods for extruding an elastomeric mixture

Info

Publication number: CN116635206A
Application number: CN202180075516.0A
Authority: CN
Inventors: R·雷加利亚
Original assignee: Pomini Rubber & Plastics LLC
Current assignee: Pomini Rubber & Plastics LLC
Priority date: 2020-11-09
Filing date: 2021-11-05
Publication date: 2023-08-22
Also published as: US20230405908A1; JP2023551115A; EP4240583A1; WO2022097076A1

Abstract

A screw (21; 22) for a twin-screw assembly of an extruder for elastomeric compounds, comprising a threaded portion with a single-start thread, which defines at least three distinct sections (20, 30, 40) of the screw from upstream to downstream in an axially extending longitudinal direction (X-X), wherein the at least three sections comprise: an inlet section (30) for capturing the mixture supplied from the outside and pushing it downstream in the longitudinal direction (X-X), the inlet section having a cross section (S) comprising a through-flow channel between adjacent flanks of the thread, the cross section being constant over at least two pitches of the thread or over 720 degrees of rotation; a transition section (40) downstream of the inlet section, the transition section having a cross-section (S) of a through-flow channel which is variable and smaller than the cross-section of the through-flow channel of the inlet section and is designed such that the advancing pressure acting on the mixture conveyed in the longitudinal direction (X-X) is increased; a high-pressure section (50) downstream of the transition section, which has a cross-section of the through-flow channel, which is minimal, constant over at least one pitch, and which is designed such that the mixture is compressed in order to obtain a maximum pressure of the mixture.

Description

Screw and twin screw assembly for use in an elastomeric mixture extruder and related methods for extruding an elastomeric mixture

The present application relates to a flighted screw for use in a twin screw extruder having intermeshing screws for extruding and/or filtering elastomer based mixtures, a twin screw extruder having intermeshing screws for elastomer mixtures and a method for extruding elastomer based mixtures.

Elastomeric materials (e.g., based on an elastomer mixture or compound) are known to be amorphous materials having a glass transition temperature below ambient temperature. In other words, the elastomeric material is a highly viscous (viscoelastic) fluid at a temperature greater than or equal to ambient temperature, which is already "rubbery" and therefore does not need to be melted for further processing.

Such materials, commonly referred to as rubber, include, for example, natural rubber, polybutadiene, polyisoprene, EPDM, NBR, and SBR.

It is known in the art that it is necessary to "filter" an elastomer-based compound in the art that it is involved in the production of such compounds; by this operation, the treated material is caused to flow through a "filter" by means of a suitable machine, which filter is typically composed of one or more metal mesh screens having mesh flow apertures of suitable size.

Typically, the pore size ranges between 0.1mm and 1 mm.

The purpose of this operation is to retain any possible "bodies" (impurities, particles of unmixed material, etc.) in the filter and thus to eliminate from the compound, the "bodies" being of a size greater than the through-flow aperture of the filter mesh.

A typical example is a compound for producing some visible profile of an automobile (e.g. window seals), for which a "perfect" surface appearance is an important feature, which requires eliminating any possible sources of surface non-uniformities from the compound used for this purpose.

Another area where filtration is important is in the field of compounds for power supply cables, which must be completely free of any impurities, in particular metallic impurities.

In order to filter the compound, it must be "pushed" or "forced" through the filter, an operation which is only possible when the compound is in a fluid state, i.e. has a predominantly "viscous" component compared to an "elastic" component, which state is created when the compound is not crosslinked (unvulcanized) so that the polymer chains are not chemically bonded together.

In the non-crosslinked (so-called "as-is") state, the elastomeric compound may be considered a "fluid" capable of "flowing" or "free movement".

Even in this state, the fluid has a relatively very high viscosity, so that during the flow movement of the compound, an undesirable temperature rise sometimes occurs due to the high friction inside the material.

Therefore, filtration must be performed under conditions where the compound is not crosslinked and the compound is absolutely unable to crosslink during filtration; thus, filtration is greatly constrained by two major factors: temperature and pressure.

Therefore, an increase in temperature (e.g., due to friction) must be avoided throughout the process. Therefore, the temperature must be controlled by reducing friction and/or by efficiently dissipating the generated heat.

The pressure value required to supply the compound during processing is also dependent on the velocity of the "fluid" passing through the filter. In all other conditions being equal, in order to obtain an increase in productivity and thus in flow, it is necessary to increase this speed precisely, thus increasing the pressure and thus the temperature.

It is also known that in various treatments involving elastomers subjected to velocity gradients, it is due to these "gradients" that mechanical stresses are generated inside the treated material, which stresses can generally cause undesired characteristic changes (for example, a decrease in viscosity due to mechanical breakage of the macromolecules).

Thus, from a rubber technology point of view, there is a problem of filtering elastomer-based compounds:

at high flow/productivity conditions;

limiting the temperature rise of the compound to a relatively low value and avoiding any initial crosslinking in any case;

avoiding any degradation of mechanical properties in the material, such as undesired breakage of the polymer chains.

From an industrial point of view, the filtration process must generally ensure:

economic sustainability of the treatment operation, and therefore its cost is low (i.e., mechanical, labor, and energy costs);

the operation is easy and safe;

the equipment layout is simple, and the automation degree is high

"environmental" sustainability, i.e., emissions and disposal waste materials with a defined low value.

In different technical fields involving the treatment of plastic compounds (for example PVC and PP), different types of single-screw or multi-screw extruders are known.

The plastic compound has a glass transition temperature above ambient temperature and must therefore be introduced into the extruder in the solid state and heated in the extruder in order to be "melted" and thus handled. The process and extruder designed for plastic materials are therefore incompatible with the correct treatment of the elastomeric compound, in which the heating must be avoided as much as possible. Thus, these two technical fields are considered to be very different from each other.

For example, known extruders for processing plastic materials with heating of the compound are described in US20150184655A1 and US2508495, which US20150184655A1 proposes the use of a profiled constant pitch screw, and US2508495 proposes the use of a screw whose pitch and width vary continuously and gradually between the compound inlet and outlet.

GB1359672 describes a single-screw or twin-screw extruder having non-intermeshing screws for processing solid plastic material (e.g. PET) and having heating means for melting the plastic material, wherein the volume comprised between adjacent crests of the screw flights is varied by means of "lands" protruding from the bottom of the flights. The raised platform does not allow the use of screws in a twin screw extruder with intermeshing screws for processing elastomeric compounds.

Such screws and extruders are not suitable for extrusion and filtration of elastomeric compounds because they have: heating means (heat transfer oil or electric resistance) designed to heat the treated plastic material to the high temperature required for melting (which would damage the elastomeric material, which, on the contrary, must not melt); and non-intermeshing screws (resulting in play between the screws, and thus excessive loss of flow, so that sufficient pressure for extrusion/filtration of the elastomer cannot be achieved). Moreover, the non-intermeshing screws operate along substantially separate and independent compound flow channels.

The means for filtering the elastomeric compound comprises known intermeshing twin/multi-screw extruders, which are usually counter-rotating and comprise screws arranged to generate a high pressure compound at the filter.

However, these known extruders have handling problems because:

when the compound is loaded into the extruder at ambient pressure, a large amount of air may remain trapped in the compound and may remain "trapped" in the compound until the filtration stage, resulting in the final product having bubbles inside it and being difficult/impossible to use for downstream filtration processes;

they have a relatively high screw length for reducing the counter flow against the main movement and obtaining the desired pumping effect. However, this length is disadvantageous for the purpose of controlling the temperature rise of the compound, since the heat generated by friction between the compound and the screw surface and the cylinder will increase with an increase of said surface.

Moreover, the larger the contact area with the compound, the greater the likelihood that the nature of the treated compound will change significantly, due to friction between the compound and the surface.

The technical problem addressed is therefore to overcome or at least reduce the drawbacks of the prior art by providing in particular a screw with improved design, suitable for a twin-screw extruder for elastomer mixtures, with axial discharge, in which they are arranged intermeshed with parallel rotation axes.

A particular problem posed is the creation of a screw geometry that enables the air present in the elastomer mixture to be expelled before it is filtered and/or that enables a reduction in the increase in temperature and in the physical properties of the treatment mixture.

The technical problem posed is also to provide a twin-screw extruder for elastomer mixtures, which:

having a loading system operating at ambient pressure; and/or

Materials that do not require lubrication treatment (i.e., no mixture loss); and/or

Has high productivity; and/or

The heat exchange is ensured to be improved, namely less heat is generated; and/or

Having a screw design to ensure high pressure only in the region of interest and along a relatively short section; and/or

The ability to expel air that is still trapped inside the mixture before it reaches the filtration zone; and/or

Without causing any change in the physical properties of the mixture.

Regarding this problem, it is also desirable that the twin screw assembly and/or extruder should be easy and inexpensive to produce and assemble, have small dimensions, and be capable of being easily installed at any user location.

According to the application, these results are obtained by means of a screw according to the features of claim 1 and a twin-screw assembly according to claim 9.

The application also relates to a method for extruding an elastomer mixture according to claim 20.

Further details can be obtained from the following description of non-limiting examples of embodiments of the subject matter of the present application, provided with reference to the attached drawings, wherein:

fig. 1: a side view of the screw according to the application is shown, wherein three different longitudinal sections of the screw are highlighted;

fig. 2: a side view of the screw according to fig. 1 is shown, in which various characteristic parameters are shown;

fig. 3: an exploded view of a twin screw assembly according to the present application is shown;

fig. 4: a perspective view of the assembled twin screw assembly according to fig. 3 is shown with the receiving cylinder opening;

fig. 5: a perspective view of the extruder of the present application assembled with a twin screw assembly according to fig. 3 is shown;

fig. 6: a schematic cross-section of an extruder according to the application is shown, in which the play present between the screws and the cylinders is shown;

fig. 7a,7b: a schematic representation of the play present between the screws and a corresponding view of the pressures obtained in the various sections;

fig. 8a-8c: a perspective view and a partial side view of a pair of screws of a twin screw assembly are shown with a C-shaped chamber highlighted and showing the different flow rates involved;

fig. 9: a view showing an example of the percentage change in the volume of the C-shaped chamber, which depends on the axial position of the chamber in the passage from the mixture inlet zone to the high pressure/output zone of the extruder;

fig. 10: a view showing an example of the progress of the actual flow along the rotation axis of the screw and the volume change of the C-shaped chamber in a preferred embodiment of the twin screw assembly of the present application, wherein the local width (W) of the crests of the threads is indicated along the X-axis;

fig. 11: a schematic cross-sectional view showing the through-flow channel of the screw according to the present application as the rotation angle of the screw varies;

fig. 12a to 12e: an example of the geometry of a screw according to the application is shown;

fig. 13a to 13e: an example of intermeshing and counter-rotating screw pairs in a mirror image arrangement, each screw pair being formed by a respective screw according to fig. 12;

fig. 14A-14E: an example of the characteristic development of the through-flow cross section of a twin-screw assembly according to fig. 13 is shown, depending on the thread winding angle.

As shown, provided for ease of illustration only and not by way of limitation, that a set of three reference axes, the longitudinal direction X-X corresponds to the axial length dimension of the screws and the direction of feed of the mixture, the transverse direction Y-Y corresponds to the radial width dimension of the screws, and is parallel to the inter-axis plane between the axes of rotation of the two screws when used in a twin-screw assembly; the vertical direction Z-Z is perpendicular to the other two directions, the screw according to the application having a threaded portion with a single-start type thread, the thread being raised with respect to the core and defining three distinct longitudinal sections, namely 30, 40 and 50 respectively.

With reference to fig. 2 and 6, some characteristic parameters of the screw according to the application are introduced below and will be referred to in the continuation of the description.

P=pitch of the screw, measured as the axial distance between the centerlines of two crests of the thread, which are arranged at a distance of 360 ° relative to each other (one complete revolution of the thread about the axis of the screw). In the application of the present application, the pitch is generally constant along the entire screw and preferably coincides with the outer diameter D;

d = outside diameter of screw. Typically constant along the entire screw;

d = inner diameter of the screw, corresponding to the diameter of the core; d may vary along the length of the screw but is preferably constant in the inlet and high pressure sections, preferably also in the transition section;

flow channel: comprising free volume between adjacent flanks of the thread (grooves corresponding to the thread)

Crest of tooth: top surface connecting two consecutive sides

Flow cross section (of channel): a cross-section of the through-flow channel (or groove) along an axial plane passing along the axis of rotation of the screw;

w = width measured in the axial direction of the crests of the threads;

h = height of the mixture flow channel;

l=length of threaded portion of screw

The profile of the thread may preferably have a trapezoidal or flattened triangular form.

To illustrate a twin screw assembly according to the present application, the following further definitions are provided:

l = interaxial distance between screws of the screw assembly (fig. 6);

σ = distance between the crest of one screw and the core of the other screw (fig. 6);

δ=the distance between the crest of one screw and the inner surface of the housing cavity containing the cylinder (fig. 6).

C-shaped chamber: the C-shaped chamber is defined by the free volume between the screws inside the containing cylinder and is comprised in a single rotation of the threads of a single screw (in other words the "pitch").

Flow-through channels (flowing): the free volume between the screws inside the receiving cylinder, which defines the mixture flow channel. The flow channel for the flow is formed by the combination of all C-shaped chambers of the two screws.

Referring to fig. 3 and 4, there are also defined an upstream portion M corresponding to the zone for the entry of the mixture to be filtered and a downstream portion V corresponding to the zone of output for the filtered mixture, the twin-screw assembly according to the application essentially comprising:

a "cylinder" 10, the cylinder 10 having a body 11, the body 11 having a top upstream opening 13 for the entry of the mixture and a downstream axial outlet opening 12; advantageously, the cylinder may be divided into two half-cylinders 11a, 11b to facilitate assembly.

The cylinder has a suitable internal cavity shaped to accommodate two screws 21 and 22, the screws 21 and 22 being arranged such that their axes of rotation are parallel, intermeshed and counter-rotated in use.

An output zone 60 is disposed at the downstream end of the twin screw assembly, said zone comprising a filtration zone 70 (fig. 5), the mixture passing through a filter (not shown) into the filtration zone 70 in a feed direction towards the discharge outlet.

For ease of description, the present description will always refer to a twin screw assembly in which the screws are arranged in mirror image with respect to each other, but different configurations of two intermeshing and counter-rotating screws of the twin screw assembly according to the present application are also contemplated.

The cylinder and screw assembly define three different sections along the longitudinal direction of the twin screw assembly (fig. 3), corresponding to the three longitudinal sections of the threaded portion of the screw, namely:

an inlet or upstream section 30 in which the mixture is introduced into the cylinder 10 at ambient pressure and "captured" by rotation of the screws 21, 22;

an intermediate or transition section 40, the intermediate or transition section 40 being located downstream of the inlet section 30, wherein the action of the two screws 21, 22 causes a gradual increase in the advancing pressure of the downstream advancing mixture;

a high-pressure section 50, which high-pressure section 50 is located between the transition section and the subsequent output region 60.

As will become more apparent below, along the mixture inlet section 30, there is a large amount of play between the screws so as to create a large "free" volume that can facilitate entry of a large amount of mixture, along the transition section 40, there is a progressively smaller play between the screws, and along the high pressure section 50, there is a very small play between the screws, i.e., less than the play of the inlet section 30 and the transition section 40. So as to create a smaller through-flow channel or free volume which minimizes reverse flow and achieves the high pressures required for filtration.

The three longitudinal sections of the screw can be determined from the variation of the through-flow channel formed in the free volume inside the cylinder with respect to the rotation angle.

In this context, reference will also be made to the known concept of a "C-shaped chamber" (fig. 8 a-8C) for identifying a channel Cx (fig. 8C) having a free volume in the form of C, defined between the screws of a pair of intermeshing screws, and included in a single turn (in other words "pitch") of the threads of a single screw.

In detail, according to the application, the twin-screw assembly of the application is characterized by a single through-flow channel comprising, from upstream to downstream, at least three distinct sections (20, 30, 40) of the screw in the longitudinal direction (X-X), comprising:

an inlet section (30), the inlet section (30) having a sufficient volume of C-shaped chambers C1-C3 forming a flow channel, it being optimized to capture the mixture supplied from the outside and to push it downstream in the longitudinal direction (X-X), and it being kept constant over at least two pitches of the flights of each screw;

a transition section (40) downstream of the inlet section, the transition section (40) having a variable volume C-shaped chamber C1-C3, thereby reducing (in the direction of the advancing movement X-X) and a C-shaped chamber volume smaller than the inlet section 30, in order in particular to create an increase in the advancing pressure acting on the conveyed mixture in the longitudinal direction X-X, and to expel air that remains trapped while loading the mixture;

a high pressure section 50, the high pressure section 50 being arranged downstream of the transition section 40 and having the volume of a C-shaped chamber C6, C7, the C-shaped chamber C6, C7 forming a through-flow channel which is constant at least one pitch and smaller than the volumes of the inlet section and the transition section; thus, this section 50 of the C-shaped chamber with the smallest volume is optimized in order to compress the mixture and obtain the maximum pressure of the mixture in the output area 60.

According to a preferred embodiment of the application, the geometric configuration of the three longitudinal sections of each screw is developed so as to produce a variation law of the through-flow channels and therefore of the C-shaped chambers, which guarantees an optimal configuration of the specific functions intended for each section. In particular, the geometry is such as to maximize the inlet performance at atmospheric pressure simultaneously, corresponding to the inlet section required for a C-shaped chamber with a large volume (hence low crest width W), thereby maximizing the flow rate and maximizing the compression of the mixture in the high pressure section 50 disposed immediately upstream of the filtration zone.

In particular, in a preferred embodiment of the screw used in the twin-screw assembly according to the application, the inlet section has the following parameters:

w= (0.025-0.20) D, preferably (0.05-0.10) D, constant for at least two, three or four pitches;

p is constant, preferably = D;

sigma= (0.0025-0.030) D, preferably (0.005-0.015) D

Delta= (0.0025-0.030) D, preferably (0.005-0.015) D

H= (0.3-0.8) D, preferably (0.54-0.6) D

Axial length of Li inlet region) = (3-4) D

The preferred geometry of the screw in the inlet section is such that in a single flight screw:

the relatively low W value and the high H value enable the maximum volume of the C-shaped chamber to be obtained;

the lower values of σ and δ and the axial transmission value (p=d) allow the mixture "capture" performance at the extruder input and mixture feed to be maximized.

In order to generate the required pressure only in the high-pressure section immediately adjacent to the filter, in order to reduce heat generation and back-flow losses, the C-shaped chamber in this region has a smaller constant volume than in the other regions of the screw, so that the play is as small as possible mechanically.

In detail, the high pressure section preferably has the following parameters:

w= (0.3-0.4) D, preferably (0.33-0.37) D, constant for one or both pitches;

p is constant, preferably = D;

sigma= (0.0025-0.020) D, preferably (0.005-0.015) D

δ= (0.0025-0.030) D, preferably (0.005-0.015) D;

h= (0.3-0.8) D, preferably (0.54-0.6) D;

lp (axial length of high-pressure section) =1-2D;

normally constant flow cross section

The geometry of the high pressure region is such that:

the lower values of δ and σ and the higher value of W maximize pumping performance, i.e. the ratio between the main flow (in the feed direction) and the counter flow opposite to the main movement;

in the case of high H, the value of p=d simultaneously maximizes the flow.

Thus, the screws of the twin-screw assembly according to the application may preferably be arranged to obtain simultaneously: the high pressure in the section 50 near the filter, where the play between the screws and the cylinder is relatively very small (in order to reduce the counter flow against the main movement); and high capture and flow of the mixture in the inlet region due to the higher free volume (i.e., the space that can potentially be filled with the mixture).

In view of the different performance characteristics required for the two sections (i.e. the inlet section at upstream/ambient pressure and the downstream/high pressure section), the transition section between the inlet section (with low pressure and high volume C-shaped chambers) and the high pressure section (with low volume C-shaped chambers upstream of the filtration zone) is preferably arranged with a variable C-shaped chamber volume, in particular so as to:

eliminating air remaining trapped in the mixture as it enters the extruder;

abrupt and unexpected changes in geometry are avoided, which cause them to create potential non-uniformities and potential local pressure peaks in the processed material.

Thus, preferably, the transition section 40 has a reduced C-shaped chamber volume. In particular, the cross-section of the flow channel of the mixture preferably decreases in the direction of the advancing movement of the mixture according to a law of variation which is substantially continuous, in particular at least locally approximately linear and/or quadratic and/or of an order greater than 2.

Preferably, the variation of the mixture flow channel in the transition section is obtained by varying the geometry of the crest width W of the screw flights, while other parameters of the screw may be kept constant in the transition section.

According to a particularly preferred geometry of the screws used in the twin-screw assembly of the present application, the transition section has the following parameters:

w=can be continuously varied between the W value of the inlet section and the W value of the high pressure section

P is constant, preferably = D;

sigma= (0.0025-0.030) D, preferably (0.005-0.015) D

δ= (0.025-0.030) D, preferably (0.005-0.015) D;

h= (0.3-0.12) D, preferably (0.54-0.6) D;

lt (axial length of transition section) =1-3D

Thereby a gradual transition between inlet performance and high pressure performance is obtained. The variation of the through-flow channel may follow a suitable law in order to optimize the desired performance.

It is particularly preferred to limit the axial length of the flighted portion of the screw in order to obtain a ratio L/D.ltoreq.8, i.e. a shorter length, in order to limit the undesirable temperature rise typical of long extruders (L/D >10 is known in the art).

According to the above law, a screw is provided having a geometry obtained on the basis of the law of variation of the internal channels and designed to determine three of said distinct treatment zones.

Referring again to fig. 8C and the mixture flow channel defined by the C-shaped chambers of the twin screw assembly, it can be seen that when there is no flow loss, the theoretical maximum flow Qth of a twin screw extruder with single flighted, intermeshing and counter-rotating screws arranged in mirror image will be equal to the volume Vc of the two C-shaped chambers multiplied by the rotational speed N of the screws:

maximum theoretical flow rate: qth=2·vc·n

Vc = C-shaped chamber volume

N＝rpm

Moreover, it is known in practice that the flow losses due to the play between the screws and the cylinders and the countercurrent flow opposite to the main movement never reach the theoretical maximum flow, and that the larger the play, the higher the flow losses, which also depend on the pressure development between the loading zone and the filtering zone.

Still referring to fig. 8C, the main flow loss is as follows:

qc=calendar (calendar) leakage

Qt = tetrahedral leakage

Qf = flight clearance leakage

Qs = side gap leakage

The effective flow Q produced is thus obtained from the algebraic sum of the theoretical maximum flow (Qth) directly derived from the volume of the C-shaped chamber and of the total flow loss (Ql) due to the play between the screws and the cylinders and to the backflow due to the pressure:

total loss of flow/reverse flow:

Ql＝Qf+Qs+Qt+Qc

effective flow rate: q=qth-Ql

According to a preferred configuration of the twin-screw assembly, said assembly is arranged to obtain a substantially constant effective flow Q along the longitudinal direction of the advancing movement of the mixture from the input section to the end of the high-pressure section, and therefore the efficiency is calculated as the ratio Q/Qth, which increases gradually towards the output zone.

A particularly preferred example of such a configuration is shown in fig. 9, which shows the geometry of the counter-rotating screw and the variation of the associated C-shaped chamber forming the mixture flow channel; the corresponding effective flow rate q=qth-Ql through the flow channel, which is constant in the extrusion direction, is indicated by the black dashed line in the view of fig. 10.

Fig. 11 shows a schematic view of a cross section S of a through-flow channel when the rotation angle of the flights of the screw according to the application is varied.

A further preferred embodiment of a screw and a corresponding twin-screw assembly according to the present application is shown in fig. 12, 13, respectively.

The cross-sectional variation of the mixture flow channels for the corresponding twin screw assemblies is shown in fig. 14.

In more detail, the screw of fig. 12a has a cross section of the through-flow channel which is constant (and maximum) over three 360 ° rotations (3 l) (three pitches) of the thread in the inlet section 30, decreases over three rotations of the thread in the transition region, and is constant (minimum) over two pitches (2P) in the high pressure region.

The C-shaped chamber and cross-section of the mixture through-flow channel for the twin screw assembly according to fig. 13a follows a similar law of variation as shown in fig. 14 a.

The screw of fig. 12b has a cross-section of the through-flow channel that is constant (and maximum) over four 360 deg. rotations of the screw thread in the inlet section 30, decreases over one 360 deg. rotation of the screw thread in the transition region, and is constant (minimum) over two pitches in the high pressure region. The C-shaped chamber and cross-section of the mixture through-flow channel of the twin screw assembly according to fig. 13b follows a similar variation law, as shown in fig. 14 b.

The screw of fig. 12c has a cross section of the through-flow channel which is constant (and maximum) over four 360 ° rotations of the thread in the inlet section 30, decreases over two pitches (2L) of the thread in the transition region, and is constant (minimum) over two pitches in the high pressure region. The C-shaped chamber and cross-section of the mixture through-flow channel of the twin screw assembly according to fig. 13C follows a similar variation law, as shown in fig. 14C.

The screw and twin screw assembly in fig. 12d, 13d, 14d is similar to the screw and twin screw assembly in fig. 12c, 13d, but in this case the change in cross section in the transition section 40 has a reduced secondary order (2L).

The screw of fig. 12e has a cross section of the through-flow channel that is constant (and maximum) over four 360 deg. rotations (three pitches) of the screw flights in the inlet section 30. The three rotations of the thread in the transition zone decrease and are constant (minimal) over 360 deg. rotation of the thread in the high pressure zone 50.

The C-shaped chamber and cross-section of the mixture through-flow channel for the twin screw assembly according to fig. 13e follows a similar variation law as shown in fig. 14 e.

Advantageously, all the preferred examples shown (these preferred examples are only some of the possible geometries) are able to keep the length of the threaded portion of the screw within ten pitches, preferably within eight pitches.

In addition to this, it is also possible to:

high pressures (even higher than 300 bar) are reached with a screw length and limited ratio L/d=length/diameter of transition zone+pressure zone equal to 5 and high flow rates, so that filtration can also be carried out using filters with very fine mesh (< 0.1 mm);

controlling and limiting the temperature rise of the mixture at high RPM and flow;

a relatively low driving torque is used because the screw is short;

preventing significant bending of the screws, preventing them from coming into contact with the cylinders;

air is expelled through the inlet opening, which air may remain trapped as the mixture enters.

In a preferred embodiment of the extruder according to the present application, it is envisaged that the extruded mixture filtration/output section 70 comprises a filter holder plate 71, which filter holder plate 71 is connected to the connecting flange 60 and is closed by a shaped head 73.

A "filter" (not shown here), typically comprising one or more metallic mesh, is arranged between the connection flange 61 and the filter holder plate 71; the mixture is pushed by the thrust created by the rotation of the screw to flow through the filter, which retains any impurities larger than the mesh.

Preferably, one or more pressure and temperature sensors 61 are arranged in the flange 60 and are capable of continuously monitoring the pressure and temperature of the treated mixture in order to obtain complete control of the filtration step.

Fig. 6, 7 show cross-sections through the housing of screws 21 and 22 in cylinder 10.

It can be seen how the play delta between the crests of the screws and the cylinder 10 and the play sigma between the crests of one screw and the core of the other screw are made very small and in any case the pumping action and the absence of contact between the screws and the cylinder can be ensured at the same time.

It is therefore clear how the screw, the twin-screw assembly and the extruder provided with the twin-screw assembly according to the application provide a solution to the problems of the prior art, resulting in:

no other auxiliary means for loading the elastomer mixture to be filtered at ambient pressure are present;

high pressures are generated in small defined areas, limiting the rise in temperature, which can advantageously be kept below the vulcanization temperature of the elastomer mixture (generally less than 100-120 ℃);

the overall performance is optimized due to the three areas, each area is dedicated to an accurate task and is set in a related manner;

due to the special geometry of the transition region, there is no local temperature peak;

as the mixture is introduced into the inlet region, air trapped in the mixture is effectively removed.

While the present application has been described in connection with various embodiments and preferred examples thereof, it is to be understood that the scope of protection of this patent is to be determined solely by the claims that follow.

Claims

1. A screw (21, 22) suitable for use in a twin screw assembly with intermeshing screws of an elastomeric mixture extruder, the screw comprising a flight portion with single flight defining different at least three sections (30, 40, 50) of the screw from upstream to downstream in an axially extending longitudinal direction (X-X), wherein the at least three sections comprise:

an inlet section (30) for capturing the mixture and pushing it downstream in a longitudinal direction (X-X), a cross section (S) of the through-flow channel of the inlet section being comprised between adjacent flanks of the thread, said cross section being constant over at least two pitches of the thread or over 720 degrees of rotation;

-a transition section (40) downstream of the inlet section, the cross-section (S) of the through-flow channel of which is variable and smaller than the cross-section of the through-flow channel of the inlet section, and designed such that the advancing pressure acting on the mixture conveyed in the longitudinal direction (X-X) is increased;

a high-pressure section (50) downstream of the transition section, the cross section of the through-flow channel of which is minimal, constant over at least one pitch, and is designed such that the mixture is compressed in order to obtain the maximum pressure of the mixture.

2. The screw according to claim 1, wherein: the cross-section of the through-flow channel and/or the width (W) of the thread crests of the inlet section is constant over at least two, preferably over at least three, in particular over two, three or four thread pitches.

3. The screw according to claim 1 or 2, characterized in that: in the inlet section, the geometry of the screw has:

crest width w= (0.025-0.20) D, preferably (0.05-0.10) D, wherein D is the outer diameter of the screw, said crest width W being constant over at least two, three or four pitches; and/or

The pitch P of the thread, which is constant, preferably equal to the outer diameter D; and/or

The height H of the through-flow channel,

h= (0.3-0.8) D, preferably (0.54-0.6) D; and/or

The axial length li= (3-4) P and/or (3-4) D of the inlet region.

4. A screw according to claim 1, 2 or 3, wherein: in the high-pressure section (50), the geometry of the screw has:

crest width w= (0.3-0.4) D, preferably (0.33-0.37) D, constant over one or both pitches; and/or

Pitch P is constant, preferably = D;

wherein D is the outer diameter of the screw.

5. A screw according to any preceding claim, wherein: the screw has a constant pitch throughout the threaded portion.

6. A screw according to any preceding claim, wherein: the length of the threaded portion of the screw is less than or equal to 10D and/or less than or equal to 10P, where D is the outer diameter of the screw and P is the pitch of the threads.

7. A screw according to any preceding claim, wherein: in the transition section, the variation of the through-flow channel for the passage of the mixture is obtained by varying the geometry of the crest width (W) of the screw flight.

8. Screw according to the preceding claim, wherein: in the transition section, the crest width W may continuously vary between a minimum value of W corresponding to the crest width of the inlet section and a maximum value of W corresponding to the W value of the high pressure section; wherein the geometry of the screw in the transition section preferably has:

the pitch P is constant, preferably equal to the outer diameter D; and/or

The height H of the through-flow channel is preferably constant and/or comprised between (0.3-0.12) D, preferably (0.54-0.6) D; and/or

The axial length lt=1-3D of the transition section;

wherein D is the outer diameter of the screw.

9. Twin screw assembly (10, 20) for an extruder for elastomeric compounds, comprising two screws (21, 22) with a threaded portion having a single-start thread, the two screws being arranged to intermesh and counter-rotate inside a cylinder (10) with parallel longitudinal rotation axes (X-X) so as to form a through-flow channel for the flow of the compound, said through-flow channel being composed jointly of a plurality of C-shaped chambers (Cx), each C-shaped chamber being defined by a free volume inside the cylinder (10) and being included in a single rotation of the threads for a single screw,

the twin-screw assembly is provided with an upstream opening (13) for letting the mixture into the through-flow channel,

the method is characterized in that: the channel for the flow through comprises, from upstream to downstream, at least three distinct sections (30, 40, 50) along the longitudinal direction (X-X) of the axial extension of the screw and of the advancing movement of the mixture, said sections comprising:

an inlet section (30) for capturing a mixture supplied from the outside and pushing the mixture downstream in a longitudinal direction (X-X), the inlet section having a C-shaped chamber volume forming a through-flow channel for flow therethrough, the C-shaped chamber volume of the inlet section being constant over at least two pitches of the flights of each screw;

a transition section (40) downstream of the inlet section, the transition section having a C-shaped chamber volume forming a flow channel, the C-shaped chamber volume of the transition section being variable, decreasing and being smaller than the C-shaped chamber volume of the inlet section;

a high pressure section (50) downstream of the transition section, the high pressure section having a C-shaped chamber volume forming a flow channel, the C-shaped chamber volume of the high pressure section being constant over at least one pitch and being smaller than the C-shaped chamber volume of the inlet section and the C-shaped chamber volume of the transition section, the high pressure section being for causing compression of the mixture so as to obtain a maximum pressure of the mixture.

10. The twin-screw assembly of the preceding claim, wherein: the screws are arranged in mirror image.

11. The twin screw assembly of any of claims 9-10, wherein: the high pressure section has a C-shaped chamber volume that is constant over at least two pitches of each screw.

12. The twin screw assembly of any of claims 9-11, wherein: the inlet section has a C-shaped chamber volume that is constant over at least three, preferably at least four, pitches of each screw.

13. The twin screw assembly of any of claims 9-12, wherein: the transition section has a C-shaped chamber volume decreasing in the direction of the advancing movement of the mixture and/or the cross-section of the through-flow channel for the flow of the mixture decreases in the direction of the advancing movement of the mixture according to a law of variation which is substantially continuous, in particular at least locally approximately linear and/or quadratic and/or has an order of more than 2.

14. The twin screw assembly of any of claims 9-13, wherein: the screw and the cylinder are arranged and configured to obtain an effective flow rate (Q) through the mixture flow channel, which is constant from the inlet section to the end of the high pressure section in the longitudinal direction of the advancing movement of the mixture.

15. The twin screw assembly of any of claims 9-14, wherein: one and/or the other of the two screws is arranged according to any of claims 1-8.

16. The twin screw assembly of any of claims 9-15, wherein:

in the inlet section:

sigma= (0.0025-0.030) D, preferably (0.005-0.015) D

δ= (0.0025-0.030) D, preferably (0.005-0.015) D; and/or

In the high pressure section:

sigma= (0.0025-0.020) D, preferably (0.005-0.015) D

δ= (0.0025-0.030) D, preferably (0.005-0.015) D; and/or

In the transition section:

σ= (0.0025-0.030) D, preferably (0.005-0.015) D;

δ= (0.025-0.030) D, preferably (0.005-0.015) D;

where σ is the distance between the crest of one screw and the core of the other screw and δ is the distance between the crest of one screw and the inner surface of the housing cavity containing the cylinder.

17. The twin screw assembly of any of claims 9-16, wherein: the upstream opening is arranged to feed the mixture to the through-flow channel in a direction (Z-Z) substantially orthogonal to the longitudinal direction (X-X).

18. An extruder for elastomer mixtures, characterized in that: the extruder comprising a twin screw assembly according to any one of claims 9-17 extending in a longitudinal direction (X-X) and comprising a downstream extrusion head (70).

19. Extruder according to the preceding claim, characterized in that: the extruder comprises a filtration zone (70) provided downstream of the high pressure zone (50) with a mesh filter through which the mixture passes.

20. A process for extruding an elastomer mixture, the process comprising the steps of:

feeding the elastomer mixture to a twin-screw assembly of a mixture extruder, said twin-screw assembly comprising two screws (21, 22) having a threaded portion, which are arranged to intermesh and counter-rotate inside a cylinder (10) with parallel longitudinal axes of rotation (X-X) so as to form a through-flow channel for the passage of the mixture flow from upstream to downstream, said through-flow channel being composed jointly of a plurality of C-shaped chambers, each C-shaped chamber being defined by a free volume between the respective screws inside the cylinder and being included in a single rotation of the threads for a single screw;

the elastomer mixture is fed through a mixture inlet opening (13) into a through-flow channel for the mixture flow through the twin-screw assembly;

capturing the elastomer mixture in an inlet section (30) of the twin screw assembly and pushing the elastomer mixture downstream through a flow channel for the mixture flow, the inlet section (30) having a C-shaped chamber volume forming a flow channel, the C-shaped chamber volume being constant over at least two pitches of the flights of each screw;

advancing the elastomer mixture through a transition section (40) of the through-flow channel for the passage of the mixture flow, said transition section (40) being arranged downstream of the inlet section in the longitudinal direction (X-X) of the axial extension of the screw and of the advancing movement of the mixture and having a C-shaped chamber volume forming the flow channel, the C-shaped chamber volume of said transition section being variable, decreasing and being smaller than the C-shaped chamber volume of the inlet section;

advancing and compressing an elastomer mixture in a high pressure section (50) arranged downstream of the transition section, the high pressure section having a C-shaped chamber volume forming a flow channel, the C-shaped chamber volume of the high pressure section being constant over at least one pitch and being smaller than the C-shaped chamber volume of the inlet section and the C-shaped chamber volume of the transition section so as to cause compression of the mixture and obtain a maximum pressure of the mixture;

the elastomer mixture passes through an extrusion head (60) arranged downstream of the high-pressure section (50) at maximum pressure.

21. The method according to the preceding claim, further comprising: the mixture being processed is filtered (70) such that the mixture passes through a mesh filter downstream of the high pressure region (50).

22. The method according to the preceding claim, wherein: the effective flow rate of the mixture through the mixture flow channel is substantially constant from the inlet section to the end of the high pressure section in the longitudinal direction of the advancing movement of the mixture.